A massively multiplayer online (MMO) role-playing game is an online computer game in which a large number of players interact with one another in a virtual world. As in most role-playing games, players assume the role of a fictional character (i.e., a Player Character) and take control over most of that character's actions. MMO games are distinguished from single-player or small multi-player games by the game's persistent world, usually hosted by a game provider, which continues to exist and evolve even when the player is away from the game.
The Player Character typically spends most of its time improving itself through the collection of experience points, which are earned, for example, through killing monsters and completing adventures assigned to the Player Character by server-controlled Non-Player Characters or Creatures (NPCs) scattered throughout the virtual world in strategic locations. NPCs are server-controlled characters who primarily exist in order to provide information, supplies, and rewards for the Player Characters. The NPCs are non-autonomous, seldom move of their own volition outside of a strictly limited area, and their attributes do not evolve over time. Their statistics, appearance and skill sets are static, and their possessions may or may not be dynamic.
Pets are a special form of Non-Player Character which occasionally accompany certain types of Player Characters. They are server-controlled partially-autonomous characters which unlike other NPCs, possess dynamic statistics and skill sets as well as the ability to move about the game environment by following around the Player Character to whom they belong.